Controlling electronic shutter in image sensors

ABSTRACT

An image sensor includes an electronic shutter layer that drains charge away from the photosensitive regions during an electronic shutter operation. A signal is applied to the electronic shutter layer through a contact. Prior to performing an electronic shutter operation, a determination is made as to whether or not a current level in the electronic shutter layer substantially equals or exceeds a threshold current level. If the current level in the electronic shutter layer substantially equals or exceeds the threshold current level, the electronic shutter operation is not performed. The electronic shutter operation is performed if the current level in the electronic shutter layer does not substantially equal or exceed the threshold current level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is related to U.S. application Ser. No.12/770,806, entitled “Electronic Shutter Control In Image Sensors”,filed concurrently herewith; U.S. application Ser. No. 12/770,811 ,entitled “Electronic Shutter Control In Image Sensors”, filedconcurrently herewith; and U.S. application Ser. No. 12/770,818,entitled “Electronic Shutter Control In Image Sensors”, filedconcurrently herewith.

TECHNICAL FIELD

The present invention generally relates to Charge-Coupled Device (CCD)image sensors, and more particularly to interline CCD image sensors withelectronic shuttering capability.

BACKGROUND

Photosensitive regions in an image sensor accumulate charge in responseto incident light. The amount of time a photosensitive regionaccumulates charge for an image is known as an integration period. Imagesensors control the amount of time in an integration period with amechanical shutter or by performing an electronic shutter operation. Anelectronic shutter operation clears charge from the photosensitiveregions by draining the charge into an underlying substrate.

A voltage applied to the substrate is set to a first level when thephotosensitive regions accumulate charge. When an electronic shutteroperation is to be performed, the voltage is changed to a second level.An image sensor, such as an interline charge-coupled device (CCD) imagesensor, can be permanently damaged when the image sensor is exposed toextremely bright light during an electronic shutter operation. Thedamage is caused by a parasitic bipolar transistor formed within theimage sensor. FIG. 1 is a schematic of a parasitic bipolar transistorformed in a prior art interline CCD image sensor. An n-type vertical CCDchannel can become an emitter of the transistor (Vccd), a groundedp-well under the vertical CCD the base, and an n-type substrate thecollector of the transistor (Vsub).

At the center of a pixel array the p-well resistance (represented by Rin FIG. 1) is very large because the ground contacts are at the edges ofthe pixel array. This large resistance allows the bright light togenerate photocurrent at the base of the transistor faster than theresistor R can drain it away. A bright spot from the sun, for example,can raise the voltage of the base high enough to turn on the transistorand short the electronic shutter voltage to the vertical CCD channel.The high voltage on the vertical CCD channel will inject charge into thevertical CCD gate dielectric. The injected charge first causes increaseddark current in the vertical CCD. With continued bright light exposureduring multiple electronic shutter pulses, a sufficient amount of chargeis injected into the gate dielectric to cause poor charge transfer andimage lag.

SUMMARY

An image sensor includes an electronic shutter layer that drains chargeaway from the photosensitive regions during an electronic shutteroperation. A signal is applied to the electronic shutter layer through acontact. Prior to performing an electronic shutter operation, adetermination is made as to whether or not a current level in theelectronic shutter layer substantially equals or exceeds a thresholdcurrent level. If the current level in the electronic shutter layersubstantially equals or exceeds the threshold current level, theelectronic shutter operation is not performed. The electronic shutteroperation is performed if the current level in the electronic shutterlayer does not substantially equal or exceed the threshold currentlevel.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are better understood with reference to thefollowing drawings. The elements of the drawings are not necessarily toscale relative to each other.

FIG. 1 is a schematic of a parasitic bipolar transistor formed in aprior art interline CCD image sensor;

FIG. 2 is a simplified block diagram of an image capture device in anembodiment in accordance with the invention;

FIG. 3 is a block diagram of a top view of an image sensor suitable foruse as image sensor 206 in an embodiment in accordance with theinvention;

FIG. 4 illustrates a first exemplary cross-sectional view along line A-Ashown in FIG. 3 in an embodiment in accordance with the invention;

FIG. 5 depicts a second exemplary cross-sectional view along line A-Ashown in FIG. 3 in an embodiment in accordance with the invention;

FIG. 6 is a flowchart of a method for sensing current in an electronicshutter layer in an embodiment in accordance with the invention;

FIG. 7 is a flowchart of a method for performing an electronic shutteroperation in an embodiment in accordance with the invention;

FIG. 8 is a block diagram of a first control circuit suitable for usewith image sensor 206 in an embodiment in accordance with the invention;

FIG. 9 illustrates a first exemplary selector component 804 shown inFIG. 8 in an embodiment in accordance with the invention;

FIG. 10 depicts a second exemplary selector component 804 shown in FIG.8 in an embodiment in accordance with the invention;

FIG. 11 is a first exemplary block and circuit diagram of blocks 802,804, 806, and 808 shown in FIG. 8 in an embodiment in accordance withthe invention;

FIG. 12 is a second exemplary block and circuit diagram of blocks 802,804, 806, and 808 shown in FIG. 8 in an embodiment in accordance withthe invention;

FIG. 13 is a block diagram of a second control circuit suitable for usewith image sensor 206 in an embodiment in accordance with the invention;

FIG. 14 is an exemplary block and circuit diagram of blocks 1300, 1306,1310, and 1316 shown in FIG. 13 in an embodiment in accordance with theinvention;

FIG. 15 is a block diagram of a third control circuit suitable for usewith image sensor 206 in an embodiment in accordance with the invention;and

FIG. 16 is an exemplary plot of the signal VH shown in FIG. 15.

DETAILED DESCRIPTION

Throughout the specification and claims, the following terms take themeanings explicitly associated herein, unless the context clearlydictates otherwise. The meaning of “a,” “an,” and “the” includes pluralreference, the meaning of “in” includes “in” and “on.” The term“connected” means either a direct electrical connection between theitems connected or an indirect connection through one or more passive oractive intermediary devices. The term “circuit” means either a singlecomponent or a multiplicity of components, either active or passive,that are connected together to provide a desired function. The term“signal” means at least one current, voltage, charge, or data signal.

Additionally, directional terms such as “on”, “over”, “top”, “bottom”,are used with reference to the orientation of the Figure(s) beingdescribed. Because components of embodiments of the present inventioncan be positioned in a number of different orientations, the directionalterminology is used for purposes of illustration only and is in no waylimiting. When used in conjunction with layers of an image sensor waferor corresponding image sensor, the directional terminology is intendedto be construed broadly, and therefore should not be interpreted topreclude the presence of one or more intervening layers or otherintervening image sensor features or elements. Thus, a given layer thatis described herein as being formed on or formed over another layer maybe separated from the latter layer by one or more additional layers.

The terms “substrate” is to be understood as a semiconductor-basedmaterial including, but not limited to, silicon, silicon-on-insulator(SOI) technology, silicon-on-sapphire (SOS) technology, bulksemiconductor substrates, doped and undoped semiconductors, epitaxiallayers, buried layers, and well regions formed in or on a semiconductorsubstrate, and other semiconductor structures.

And finally, the term “electronic shutter layer” is to be understood asa semiconductor-based material that is used to drain charge away fromphotosensitive regions during an electronic shutter operation.

Referring to the drawings, like numbers indicate like parts throughoutthe views.

FIG. 2 is a simplified block diagram of an image capture device in anembodiment in accordance with the invention. Image capture device 200 isimplemented as a digital camera in FIG. 2. Those skilled in the art willrecognize that a digital camera is only one example of an image capturedevice that can utilize an image sensor incorporating the presentinvention. Other types of image capture devices, such as, for example,cell phone cameras, scanners, and digital video camcorders, can be usedwith the present invention.

In digital camera 200, light 202 from a subject scene is input to animaging stage 204. Imaging stage 204 can include conventional elementssuch as a lens, a neutral density filter, an iris and a shutter. Light202 is focused by imaging stage 204 to form an image on image sensor206. Image sensor 206 captures one or more images by converting theincident light into electrical signals. Digital camera 200 furtherincludes processor 208, memory 210, display 212, and one or moreadditional input/output (I/O) elements 214. Although shown as separateelements in the embodiment of FIG. 2, imaging stage 204 may beintegrated with image sensor 206, and possibly one or more additionalelements of digital camera 200, to form a camera module. For example, aprocessor or a memory may be integrated with image sensor 206 in acamera module in embodiments in accordance with the invention.

Processor 208 may be implemented, for example, as a microprocessor, acentral processing unit (CPU), an application-specific integratedcircuit (ASIC), a digital signal processor (DSP), or other processingdevice, or combinations of multiple such devices. Various elements ofimaging stage 204 and image sensor 206 may be controlled by timingsignals or other signals supplied from processor 208.

Memory 210 may be configured as any type of memory, such as, forexample, random access memory (RAM), read-only memory (ROM), Flashmemory, disk-based memory, removable memory, or other types of storageelements, in any combination. A given image captured by image sensor 206may be stored by processor 208 in memory 210 and presented on display212. Display 212 is typically an active matrix color liquid crystaldisplay (LCD), although other types of displays may be used. Theadditional I/O elements 214 may include, for example, various on-screencontrols, buttons or other user interfaces, network interfaces, ormemory card interfaces.

It is to be appreciated that the digital camera shown in FIG. 2 maycomprise additional or alternative elements of a type known to thoseskilled in the art. Elements not specifically shown or described hereinmay be selected from those known in the art. As noted previously, thepresent invention may be implemented in a wide variety of image capturedevices. Also, certain aspects of the embodiments described herein maybe implemented at least in part in the form of software executed by oneor more processing elements of an image capture device. Such softwarecan be implemented in a straightforward manner given the teachingsprovided herein, as will be appreciated by those skilled in the art.

Referring now to FIG. 3, there is shown a top view of an image sensorsuitable for use as image sensor 206 in an embodiment in accordance withthe invention. Image sensor 300 is implemented as an interlineCharge-Coupled Device (CCD) image sensor in the FIG. 3 embodiment. Imagesensor 300 includes a number of pixels 302 typically arranged in rowsand columns that form imaging area 304. Each pixel includes aphotosensitive region 306 that collects charge carriers 308 in responseto incident light. A vertical CCD shift register 310 is positionedadjacent to each column of pixels.

To read out an image captured by the image sensor, appropriate biassignals are generated by a timing generator (not shown in FIG. 3) andapplied to transfer regions or gates (not shown) disposed between thephotosensitive regions 306 and respective shift elements 312 in thevertical CCD shift registers 310. The charge 308 in all of the verticalCCD shift registers 310 is then shifted in parallel one row at a timeinto shift elements 314 in horizontal CCD shift register 316. Each rowof charge is then shifted serially one shift element 314 at a timethrough horizontal CCD shift register 316 to output circuit 318. Outputcircuit 318 includes an amplifier (not shown) in an embodiment inaccordance with the invention.

FIGS. 4 and 5 illustrate exemplary cross-sectional views along line A-Ashown in FIG. 3 in embodiments in accordance with the invention. Thecross-section view extends horizontally through two pixels 302 and twovertical CCD shift registers 310. The exemplary embodiments shown inFIGS. 4 and 5 are described with specific conductivity types. Thoseskilled in the art will recognize that different conductivity types canbe used in other embodiments in accordance with the invention.

In the FIG. 4 embodiment, the entire structure is built on an n-typesilicon substrate 400 that functions as the electronic shutter layer. Ap-type well 402 is formed in the substrate 400 to isolate substrate 400from an n-type channel 404 in the vertical CCD 310. The flow of chargethrough channel 404 is controlled by gates 406. Gates 406 are formedwith polysilicon in an embodiment in accordance with the invention.

Channel 404 in vertical CCD 310 is also covered by an opaque lightshield 408 to prevent the photo-generation of charge directly in channel404. Openings in the light shield 408 allow light to penetrate thesilicon surface and generate charge in n-type photosensitive region 306.Photosensitive region 306 is separated from substrate 400 by a lightlydoped p-type vertical overflow drain 410.

The surface potential of photosensitive region 306 is held at a knownpotential, such as zero volts, by p+pinning layer 412. Pinning layer 412also acts as a separator between photosensitive region 306 and channel404.

Contact 414 allows a signal to be applied to substrate 400. During anelectronic shutter operation, the signal level of a shutter signalapplied to contact 414 is increased so that unwanted charge inphotosensitive regions 306 drains into substrate 400.

Referring now to FIG. 5, there is shown a second exemplarycross-sectional view along line A-A shown in FIG. 3 in an embodiment inaccordance with the invention. The structure of FIG. 5 is similar to thestructure of FIG. 4, except for n-type buried layer 500 formed in p-typesubstrate 502. Buried layer 500 acts as the electronic shutter layer inthe illustrated embodiment. Contact 504 allows a signal to be applied toburied layer 500. During an electronic shutter operation, the signallevel of a shutter signal applied to contact 504 is increased so thatunwanted charge in photosensitive regions 306 drains into buried layer500.

In the embodiments shown in FIGS. 4 and 5, the electronic shutter layeris the n-type substrate 400 and the n-type buried layer 502,respectively. With an n-type semiconductor material, the signal level ofthe shutter signal is increased during an electronic shutter operation.Those skilled in the art will recognize that the signal level of theshutter signal decreases when a p-type semiconductor material is used aselectronic shutter layer.

The methods depicted in FIGS. 6 and 7 operate concurrently in anembodiment in accordance with the invention. FIG. 6 is a flowchart of amethod for sensing current in an electronic shutter layer in anembodiment in accordance with the invention. Initially, a determinationis made at block 600 as to whether or not a current level in theelectronic shutter layer equals or exceeds a threshold current level. Ifthe current level does not equal, or does not exceed, the thresholdcurrent level, the state of an alert signal is changed or set to a firstlevel (block 602). If the current level does equal, or does exceed, thethreshold current level, the state of the alert signal is changed or setto a second level (block 604). The process then returns to block 600.The method of FIG. 6 operates continuously in an embodiment inaccordance with the invention.

FIG. 7 is a flowchart of a method for performing an electronic shutteroperation in an embodiment in accordance with the invention. Initially,the timing generator component produces timing signals for the imagesensor (block 700). Examples of the timing signals include, but are notlimited to, the timing signals to control the vertical and horizontalCCD shift register operations.

A determination is then made at block 702 as to whether or not anelectronic shutter operation is to be performed. If an electronicshutter operation is to be performed, a determination is made at block704 as to whether or not the state of the alert signal is at the firststate. If the state of the alert signal is not at the first state, theelectronic shutter operation is not performed (block 706) and the methodreturns to block 700.

If the state of the alert signal is at the first state, the processcontinues at block 708 where an electronic shutter operation isperformed by changing the signal level of a shutter signal from a firstlevel to a second level. When the electronic shutter operation iscomplete, the signal level of the shutter signal is returned, orchanged, to the first level (block 710). The method then returns toblock 700.

When an n-type semiconductor material is used as the electronic shutterlayer, the signal level of the shutter signal is increased at block 708and decreased at block 710. Thus, the second level of the shutter signalis greater than the first level with an n-type electronic shutter layer.Those skilled in the art will recognize that the signal level of theshutter signal can decrease at block 708 and increase at block 710during an electronic shutter operation when a p-type semiconductormaterial is used as the electronic shutter layer.

Referring now to FIG. 8, there is shown a block diagram of a firstcontrol circuit suitable for use with image sensor 206 in an embodimentin accordance with the invention. Control circuit 800 includes timinggenerator component 802, selector component 804, electronic shutterpulse driver component 806, and current sensing component 808. Timinggenerator component 802 produces timing signals for the image sensor206. For example, timing generator component 802 generates timingsignals that control the state of vertical and horizontal CCD clockdrivers 810 connected to image sensor 206.

Timing generator component 802 also produces a drive pulse signal online 812 when an electronic shutter operation is to be performed inimage sensor 206. The drive pulse signal, when received by electronicshutter pulse driver component 806, causes electronic shutter pulsedriver component 806 to change the signal level of a shutter signal froma first level to a second level. Electronic shutter pulse drivercomponent 806 produces the first signal level for the shutter signal online 814 when the photosensitive regions in image sensor 206 accumulatecharge. When an electronic shutter operation is to be performed,electronic shutter pulse driver component 806 responsively outputs adifferent second level for the shutter signal on line 814. By way ofexample only, in an embodiment using an n-type semiconductor material asthe electronic shutter layer, the first signal level is between threeand twelve volts and the second signal level is between twenty andthirty volts.

Current sensing component 808 senses a current level in the electronicshutter layer in image sensor on line 814. If image sensor 206 isexposed to damaging bright light, a large photocurrent can be present online 814. When the current on line 814 equals or exceeds a thresholdcurrent level, current sensing component 808 changes a state of an alertsignal on line 816.

Selector component 804 receives the alert signal and the drive pulsesignal produced by timing generator component 802. Selector component804 does not transmit the drive pulse signal to electronic shutter pulsedriver component 806 when the alert signal is at the second state. Anelectronic shutter operation is not performed when electronic shutterpulse driver component 806 does not receive the drive pulse signal. Thesignal level of the shutter signal remains at the first signal levelwhen electronic shutter pulse driver component 806 does not receive thedrive pulse signal. The signal level of the shutter signal remains atthe first signal level when the current level in the electronic shutterlayer in image sensor 206 equals or exceeds the threshold current level.

As discussed earlier, current sensing component 808 sets or changes thestate of the alert signal to the first state when the current on line814 does not equal, or does not exceed the threshold current level.Selector component 806 transmits the drive pulse signal to electronicshutter pulse driver component 806 when the alert signal is at the firststate. An electronic shutter operation is performed when electronicshutter pulse driver component 806 receives the drive pulse signal.

Selector component 804 can be implemented as a separate component,integrated within timing generator component 802 (see FIG. 11), orintegrated within electronic shutter pulse driver component 806 (seeFIG. 12). Additionally, timing generator component 802, selectorcomponent 804, electronic shutter pulse driver component 806, andcurrent sensing component 808, either individually or in variouscombinations thereof, can be included with image sensor 206 as amonolithic integrated circuit. Alternatively, timing generator component802, selector component 804, electronic shutter pulse driver component806, current sensing component 808, and image sensor 206, eitherindividually or in various combinations thereof, can be implemented inan image capture device as two or more separate integrated circuits.

FIG. 9 illustrates a first exemplary selector component 804 shown inFIG. 8 in an embodiment in accordance with the invention. Selectorcomponent 804 is implemented as a logic OR gate in the illustratedembodiment. The OR gate receives the drive pulse signal on input line900 and the alert signal on input line 902. The drive pulse signal isreceived by the electronic shutter pulse driver component only when thealert signal is in a low (or off) state. The OR gate does not transmitthe drive pulse signal to the electronic shutter pulse driver componentwhen the alert signal is high (or on).

Referring now to FIG. 10, there is shown a second exemplary selectorcomponent 804 shown in FIG. 8 in an embodiment in accordance with theinvention. Selector component 804 is implemented as a switch, such as amultiplexer circuit, in the illustrated embodiment. The switch receivesthe drive pulse signal on input line 1000 and a reference signal oninput line 1002. The reference signal has a constant signal level in oneembodiment in accordance with the invention.

The alert signal is received by the switch on select line 1004. Thestate of the alert signal is used to select either the drive pulsesignal or the reference signal as the output signal for line 1006. Byway of example only, the switch transmits the drive pulse signal to theelectronic shutter pulse driver component when the alert signal is in alow (or off) state. The switch produces the reference signal on outputline 1006 when the alert signal is high (or on).

FIG. 11 is a first exemplary block and circuit diagram of blocks 802,804, 806, and 808 shown in FIG. 8 in an embodiment in accordance withthe invention. The exemplary embodiment depicts specific component typesand values, and is described with particular signal levels. Thoseskilled in the art will recognize that different component types,component values, or signal levels can be implemented in otherembodiments in accordance with the invention.

Additionally, the embodiment of FIG. 11 uses a threshold current levelof approximately 1 milliamp (mA) over a range of voltages 8 to 14 volts(average of 11 volts). The threshold current level is based on the sizeof the image sensor in embodiments in accordance with the invention.Larger image sensors can require a higher threshold current level whilesmaller image sensors a lower threshold current level. The thresholdcurrent level is set to a level just below a current level thatcorresponds to a damage-causing light level in an embodiment inaccordance with the invention.

Current sensing component 808 senses the current level in the electronicshutter layer on line 814. When the current flowing through resistor1100 is less than 1 mA, transistor 1102 (Q1) is turned off and the alertsignal on line 816 is in a first state (e.g., a low state). When thecurrent flowing through resistor 1100 (R1) substantially equals or isgreater than 1 mA, transistor 1102 (Q1) turns on and pulls the alertsignal on line 816 to a second state (e.g., a high state). Diode 1104(D2) clamps the state of the alert signal to potential 1106. In theembodiment shown in FIG. 11, potential 1106 is the logic power supplyhigh for selector component 804 (OR gate logic power supply high).

Selector component 804 is integrated within timing generator component802 in the illustrated embodiment. When an electronic shutter operationis to be performed, signal generator 1108 in timing generator component802 produces the drive pulse signal on line 812. When the alert signalon line 816 is at a low state, the drive pulse signal is output byselector component 804 on line 1110 and received by electronic shutterpulse driver component 806. Shutter pulse generator 1112 in electronicshutter pulse driver component 806 changes a signal level of a shuttersignal from a first level to a second level in response to receiving thedrive pulse signal. The shutter signal is transmitted to the electronicshutter layer in the image sensor on line 814 and an electronic shutteroperation is performed. Any coupling of the shutter signal is filteredout by capacitor 1114 through diode 1116 (D1).

When the current level on line 814 equals or exceeds the thresholdcurrent level, current sensing component 808 changes the state of thealert signal to a high state. Selector component 804 does not transmitthe drive pulse signal to electronic shutter pulse driver component 806when the state of the alert signal is high. An electronic shutteroperation is not performed when the drive pulse signal is not receivedby electronic shutter pulse driver component 806. The signal level ofthe shutter signal on line 814 remains at the first level and is notchanged to the second level.

FIG. 12 depicts a second exemplary block and circuit diagram of blocks802, 804, 806, and 808 shown in FIG. 8 in an embodiment in accordancewith the invention. The function and structure of the FIG. 12 embodimentis similar to the

FIG. 11 embodiment, except that selector component 804 is integratedwithin electronic shutter pulse driver component 806.

Referring now to FIG. 13, there is shown a block diagram of a secondcontrol circuit suitable for use with image sensor 206 in an embodimentin accordance with the invention. Timing generator component 1300produces timing signals for the image sensor 206. For example, timinggenerator component 1300 generates timing signals that control the stateof vertical and horizontal CCD clock drivers 1302 connected to imagesensor 206.

Timing generator 1300 also produces a drive pulse signal on line 1304when an electronic shutter operation is to be performed in image sensor206. The drive pulse signal is received by electronic shutter pulsedriver 1306. Electronic shutter pulse driver component 1306 changes asignal level of a shutter signal on line 1308 from a first level to asecond level in response to receiving the drive pulse signal.

Current sensing component 1310 senses a current level in the electronicshutter layer in image sensor 206 on line 1312. If image sensor 206 isexposed to damaging bright light, a large photocurrent can be present online 1312. When the current on line 1312 equals or exceeds a thresholdcurrent level, current sensing component 1310 changes a state of analert signal on line 1314.

Selector component 1316 receives the alert signal on line 1314, thedrive pulse signal on line 1308, and a reference signal on line 1318.Selector component 1316 is implemented as a switch, such as amultiplexer, in an embodiment in accordance with the invention. Based onthe state of the alert signal on line 1314, selector component 1316either transmits, or does not transmit, the shutter signal to theelectronic shutter layer in image sensor 206. For example, selectorcomponent 1316 transmits the shutter signal to the electronic shutterlayer when the state of the alert signal is at the first (e.g., low oroff) state. An electronic shutter operation is performed when theshutter signal is received by the electronic shutter layer in imagesensor 206. Selector component 1316 does not transmits the shuttersignal to the electronic shutter layer when the state of the alertsignal is at the second (e.g., high or on) state. Instead, in theillustrated embodiment, selector component 1316 transmits the referencesignal to the electronic shutter layer. An electronic shutter operationis not performed when the reference signal is received by the electronicshutter layer in image sensor 206. The reference signal equals, orsubstantially equals, the first level of the shutter signal in anembodiment in accordance with the invention. Different signal levels canbe used for the reference signal in other embodiments in accordance withthe invention.

Selector component 1316 can be implemented as a separate component,integrated within electronic shutter pulse driver component 1306, orintegrated within current sensing component 1310. Additionally, timinggenerator component 1300, electronic shutter pulse driver component1306, selector component 1316, and current sensing component 1310,either individually or in various combinations thereof, can be includedwith image sensor 206 as a monolithic integrated circuit. Alternatively,timing generator component 1300, selector component 1316, electronicshutter pulse driver component 1306, current sensing component 1310, andimage sensor 206, either individually or in various combinationsthereof, can be implemented in an image capture device as two or moreseparate integrated circuits.

FIG. 14 depicts an exemplary block and circuit diagram of blocks 1300,1306, 1310, and 1316 shown in FIG. 13 in an embodiment in accordancewith the invention. In the illustrated embodiment, selector component1316 is implemented as a switch 1400. One example of a switch is amultiplexer. Switch 1400 receives the shutter signal produced byelectronic shutter pulse driver component 1306 on input line 1308 and areference signal on input line 1318. Output line 1312 is electricallyconnected to the contact to the electronic shutter layer (e.g. contact414 in FIG. 4 and contact 504 in FIG. 5).

Current sensing component 1310 operates similarly to current sensingcomponent 808 shown in FIG. 8 in an embodiment in accordance with theinvention. The alert signal produced by current sensing component 1310on line 1314 is received by switch 1400. Based on the state of the alertsignal, switch 1400 transmits either the shutter signal or the referencesignal to the electronic shutter layer on line 1312. By way of exampleonly, switch 1400 outputs the shutter signal when the alert signal is ina low (or off) state and outputs the reference signal when the alertsignal is in a high (or on) state. An electronic shutter operation isperformed when the contact to the electronic shutter layer in imagesensor 206 receives the shutter signal. An electronic shutter operationis not performed when the contact to the electronic shutter layer inimage sensor 206 receives the reference signal in an embodiment inaccordance with the invention.

Referring now to FIG. 15, there is shown a block diagram of a thirdcontrol circuit suitable for use with image sensor 206 in an embodimentin accordance with the invention. A current limited reference component1500 outputs a DC voltage signal (VSUBdc) that is transmitted to imagesensor 206 through diode 1502. VSUBdc provides a DC voltage signal online 1501 that is received by the electronic shutter layer in imagesensor 206. Current limited reference component 1500 has a transfercharacteristic such that its output voltage is constant as its loadcurrent ISUBdc varies from zero to a threshold current level ILIMIT.

Electronic shutter pulse driver component 1506 produces a shutter signalthat has a first level when the photosensitive regions in image sensor206. The first level is the same as the DC voltage produced by currentlimited reference component 1500 in an embodiment in accordance with theinvention. By way of example only, the first level can be between threeand twelve volts.

When an electronic shutter operation is to be performed, timinggenerator 1508 transmits a drive pulse signal to electronic shutterpulse driver component. The drive pulse signal, when received byelectronic shutter pulse driver component 1506, causes electronicshutter pulse driver component 1506 to change the signal level of theshutter signal from a first level to a second level. Capacitor 1510 addsthe increased voltage to the signal on line 1501 that is received by theelectronic shutter layer in image sensor 206.

Scaler component 1504 receives VSUBdc and produces a high voltage signal(VH) having a first signal level (see plot 1600 in FIG. 16). The voltagesignal VH is received by electronic shutter pulse driver component 1506.The voltage signal VH can be defined by the equation:VH=a×VSUBdc+bIn one embodiment in accordance with the invention, the values forvariables “a” and “b” are determined such that the signal level ofsignal VH is equal to the desired second level for the shutter signalfor an electronic shutter operation when ISUBdc is below ILIMIT. By wayof example only, the second level for the shutter signal is betweentwenty and thirty volts.

When the load current ISUBdc in the electronic shutter layer exceeds thethreshold current level ILIMIT, scaler component 1504 scales the levelof signal VH to a different second level. Electronic shutter pulsedriver component 1506 reduces the signal level of the shutter signal toa third level in response to receiving the scaled VH signal. The signallevel of the shutter signal decreases to the third level when the loadcurrent ISUBdc exceeds the threshold current level ILIMIT. The thirdlevel can be the same as the first level or the third level can bebetween the first and second levels.

The electronic shutter operation is not performed when electronicshutter pulse driver component 1506 receives the scaled VH signal. Inthe illustrated embodiment, the signal level of signal VH rapidlydecreases (see plot 1602 in FIG. 16) to the second level (see plot 1604in FIG. 16). The rapid decrease of signal VH protects image sensor 206from damage under high light conditions.

When the load current ISUBdc in the electronic shutter layer does notexceed the threshold current level ILIMIT, scaler component 1504transmits the VH signal at the first level to electronic shutter pulsedriver component. Note that output of current limited referencecomponent 1500 has the same function form as FIG. 16. Scaler component1504 scales the signal to the larger voltages needed for electronicshuttering. Electronic shutter pulse driver component 1506 outputs theshutter signal at the second level and an electronic shutter operationis performed in image sensor 206.

Electronic shutter pulse driver component 1506 is designed so that VH isgreater than VL in an embodiment in accordance with the invention. Theelectronic shutter drive circuits in image sensor 206 are protected fromdamage when ILIMIT is exceeded because VH is decreased to a safe valueby scaler component 1504. Alternatively, scaler component 1504 can bedesigned so that VH is greater than VL to protect the electronic shutterdriver circuits in other embodiments in accordance with the invention.

The timing generator, electronic shutter pulse driver component 1506,scaler component 1504, and current reference component 1500, eitherindividually or in various combinations thereof, can be included withimage sensor 206 as a monolithic integrated circuit. Alternatively, thetiming generator, electronic shutter pulse driver component 1506, scalercomponent 1504, current reference component 1500, and image sensor 206,either individually or in various combinations thereof, can beimplemented in an image capture device as two or more separateintegrated circuits.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention. Additionally, even though specific embodiments of theinvention have been described herein, it should be noted that theapplication is not limited to these embodiments. In particular, anyfeatures described with respect to one embodiment may also be used inother embodiments, where compatible. And the features of the differentembodiments may be exchanged, where compatible.

PARTS LIST

-   200 image capture device-   202 light-   204 imaging stage-   206 image sensor-   208 processor-   210 memory-   212 display-   214 other input/output (I/O)-   300 image sensor-   302 pixel-   304 imaging area-   306 photosensitive region-   308 charge-   310 vertical charge-coupled device shift register-   312 shift element-   314 shift element-   316 horizontal charge-coupled device shift register-   318 output circuit-   400 substrate-   402 well-   404 shift element in vertical charge-coupled device shift register-   406 gate-   408 light shield-   410 vertical overflow drain-   412 pinning layer-   414 contact-   500 buried layer-   502 substrate-   504 contact-   800 shutter control circuit-   802 timing generator component-   804 selector component-   806 electronic shutter pulse driver component-   808 current sensing component-   810 clock drivers-   812 signal line-   814 signal line-   816 signal line-   900 input line-   902 input line-   904 output line-   1000 input line-   1002 input line-   1004 select line-   1006 output line-   1100 resistor-   1102 transistor-   1104 diode-   1106 potential-   1108 signal generator-   1110 signal line-   1112 shutter pulse generator-   1114 capacitor-   1116 diode-   1300 timing generator component-   1302 clock drivers-   1304 signal line-   1306 electronic shutter pulse driver component-   1308 signal line-   1310 current sensing component-   1312 signal line-   1314 signal line-   1316 selector component-   1318 signal line-   1400 switch-   1500 current reference component-   1501 signal line-   1502 diode-   1504 scaler component-   1506 electronic shutter pulse driver component-   1508 timing generator-   1510 capacitor-   1600 plot-   1602 plot-   1604 plot

1. A method for operating an image sensor that includes an electronicshutter layer and a contact to the electronic shutter layer, the methodcomprising: prior to performing an electronic shutter operation,determining whether a current level in the electronic shutter layerexceeds a threshold current level; and when the current level in theelectronic shutter layer exceeds the threshold current level, notperforming the electronic shutter operation.
 2. The method as in claim1, further comprising performing the electronic shutter operation whenthe current level in the electronic shutter layer does not exceed thethreshold current level.
 3. The method as in claim 1, further comprisingperforming the electronic shutter operation when the current level inthe electronic shutter layer equals the threshold current level.
 4. Themethod as in claim 2, further comprising: setting a state of an alertsignal to a first state when the current level in the electronic shutterlayer does not exceed the threshold current level; setting a state of analert signal to a second state when the current level in the electronicshutter layer exceeds the threshold current level; transmitting a drivepulse signal to an electronic shutter pulse driver component when thestate of the alert signal is at the first state to cause the electronicshutter operation to be performed; and not transmitting the drive pulsesignal to the electronic shutter pulse driver component when the stateof the alert signal is at the second state to cause the electronicshutter operation to not be performed.
 5. The method as in claim 4,wherein transmitting a drive pulse signal to an electronic shutter pulsedriver component when the state of the alert signal is at the firststate causes a signal level of a shutter signal to be changed from afirst level to a second level for the electronic shutter operation. 6.The method as in claim 4, wherein not transmitting the drive pulsesignal to the electronic shutter pulse driver component when the stateof the alert signal is at the second state causes a signal level of ashutter signal to not be changed from a first level to a second level tonot perform the electronic shutter operation.
 7. The method as in claim2, wherein performing the electronic shutter operation compriseschanging a signal level of a shutter signal from a first level to asecond level.
 8. The method as in claim 7, wherein not performing theelectronic shutter operation comprises not changing the signal level ofthe shutter signal to the second level.
 9. The method as in claim 8,further comprising changing the signal level of the shutter signal to athird level.
 10. The method as in claim 8, further comprising scalingthe signal level of the shutter signal to a third level.